Other
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusmsm22a..07l&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SM22A-07
Other
7829 Kinetic Waves And Instabilities, 7851 Shock Waves (4455), 7867 Wave/Particle Interactions (2483, 6984)
Scientific paper
It has been well documented that the plasma immediately downstream of Earth's quasi-perpendicular bow shock, which consists of reflected protons and directly transmitted ions with large temperature anisotropies, is unstable to the excitation of ion-cyclotron waves. These waves in turn scatter the protons and ions to marginal stability. The wave spectrum excited by the dominant proton-wave interaction can be significantly modified by the presence of the transmitted helium. The predicted wave spectrum excited in this plasma has two humps: one corresponding to the proton instability with frequencies Ω>ΩHe ( ΩHe is the helium gyrofrequency) and the other corresponding to the helium instability with frequencies Ω<ΩHe. However, waves with frequencies Ω≅ΩHe are damped and some of the helium ions are heated perpendicular to the magnetic field due to double resonance with the ion- cyclotron waves. Using Cluster data following the inbound shock crossing at 17:17:48 on 31 March 2001, we investigate the joint evolution of the proton and helium distribution functions. The observed spectra of magnetic fluctuations, dominated by left-hand circularly polarized waves, display one hump at Ω<ΩHe just downstream of the shock, and two humps with a slot at Ω≅ΩHe further downstream. The observed temperature anisotropy of helium first increases in the near shock region before decreasing further downstream, which implies that some of the alpha particles are heated perpendicular to the magnetic field as predicted. The observations generally support the linear and quasilinear analysis. Nevertheless, certain features of the observations, such as the larger growth rate of the helium instability relative to the proton instability, require further work.
Kucharek Harald
Lee Martin A.
Liu Yong-Chun
Miao Baiqi
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